The present invention relates to liquid crystal displays using ferroelectric liquid crystal (FLC) mixtures. More precisely, the present invention relates to liquid crystal displays which contain FLC mixtures having a high spontaneous polarization and a short helical pitch and an alignment layer containing lipophilizing or lipophilic compounds (referred to as lipophilic compounds hereinafter), or amphiphilic compounds.
Switching and display devices containing ferroelectric liquid-crystal mixtures (FLC displays) are known, for example, from EP-B 0 032 362 (=U.S. Pat. No. 4,367,924). Liquid-crystal light valves are devices which, for example as a consequence of electrical actuation, modify their optical transmission properties in such a manner that incident light (which may be reflected again) is modulated in intensity. Examples are conventional watch and calculator displays or liquid-crystal displays in the OA (office automation) or TV sectors. However, these also include light shutters as are employed in photocopiers, printers, welding goggles, polarized spectacles for 3D viewing, etc. Spatial light modulators are also applications for liquid-crystalline light valves (see also Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun, Tokyo, 1989; ISBN 4-52602590-9C 3054 and the papers cited therein)
Electro-optical switching and display elements are constructed in such a manner that a liquid-crystal layer is surrounded on both sides by layers which are usually, in this sequence starting from the FLC layer, at least one alignment layer, electrodes and an outer plate (for example made of glass). In addition, they contain one polarizer if they are operated in "guest-host" or in reflective mode, or two polarizers if transmissive birefringence mode is used. The switching and displays elements may also contain further auxiliary layers, such as, for example, diffusion barrier or insulation layers.
The alignment layers, which comprise organic (for example polyimide, polyamide or polyvinyl alcohol) or inorganic (for example SiO) materials, bring, together with a spacing between the outer plates which is chosen to be sufficiently small, the FLC molecules of the FLC mixture into a configuration in which the molecules are arranged with their longitudinal axes parallel to one another and the smectic planes are disposed perpendicular or obliquely to the orientation layer. In this arrangement, as is known, the molecules have two equivalent orientations, between which they can be switched by pulsed application of an electric field, i.e. FLC displays can be switched in a bistable manner and the switching times are in the .mu.s range, these being inversely proportional to the spontaneous polarization of the FLC mixture.
The essential advantage of such FLC displays as compared with the LC displays which can still todate essentially be encountered in industrial practice is considered to be the attainable multiplex ration, i.e. the maximum number of lines which can be driven in the time-sequential process ("multiplex" process), which is significantly larger in the case of FLC displays.
The electrical drive is essentially based on the above-described pulsed addressing, which has been described by way of example in SID 85 DIGEST p. 131 (1985). In FLC displays, the smectic C* phase of the liquid-crystalline material is preferred due to its comparatively low viscosity and therefore short switching time.
In the case of the hitherto known FLC displays, the starting point is, on a controlled basis, that the spacing of the boundary plates is chosen in such a manner that the development of the twist ("helix") typical of ferroelectric phases is suppressed. This takes place in such a manner that--see the initially cited EP-B- the spacing of the plates is smaller than approximately five times the natural helical pitch. However, on account of the optical switching behaviour and for general technological reasons, there is virtually no possibility of setting the spacing of the plates to be arbitrarily small, so that in practice a spacing of approximately 2 .mu.m is adopted. For this reason, efforts were hitherto made to develop FLC mixtures having the greatest possible helical pitch; this has also recently been made subject to further demands (Gray et al., Thermotropic Liquid Crystals, 1987) according to which the spacing of the plates should be less than one quarter of the helical pitch. Examples of FLC mixtures having a high spontaneous polarization and short helical pitch are also described in JP-A 38623/91.
In order to achieve a short switching time, it is required to develop FLC mixtures having a high spontaneous polarization, because as mentioned above, switching times are inversely proportional to the value of the spontaneous polarization of the FLC mixture. It is also required to develop switching and display devices which can significantly reduce a surface memory effect and achieve a greater display contrast.
Accordingly, the object of the present invention is to provide FLC mixtures which exhibit a very low surface memory effect despite of a high spontaneous polarization. Another object of the present invention is to provide ferroelectric liquid crystal displays of improved brightness, improved contrast and improved switching time by using the above mentioned FLC mixture.